Thermally responsive switches such as circuit breakers interrupt electrical circuits in response to the occurrence of selected overload conditions in the circuits to protect other equipment from damage due to overheating, overcurrent or the like. An example of such a switch is shown in U.S. Pat. No. 4,780,697, assigned to the assignee of the present invention. As shown and described in the patent, a movable contact and a complimentary stationary contact cooperate with a control mechanism for normally holding the movable contact in engagement with the stationary contact in a closed circuit position. The control mechanism includes a current carrying thermally responsive bimetallic member having substantial electrical resistance properties adapted to conduct load current so that overload current flowing in the circuit for a selected period time self heats the bimetallic member causing it to flex and trip the control mechanism to open the circuit in a conventional manner.
It is known to use a grid system or similar structure placed adjacent to the electrical contacts in such devices in order to dissipate the energy of arcs which occur upon circuit interruption as the movable contact moves away from the stationary contact in order to minimize overheating and concomitant damage to the contacts. One example of this approach is shown and described in the above referenced U.S. Pat. No. 4,780,697 which also provides certain structures for mounting the contacts to form a selected loop to thereby electromagnetically direct arcs between the contacts away from the movable contact into an arc chute. These improvements resulted in being able to replace tungsten contact material in such circuit breakers with silver cadmium oxide for improved performance. However, even with such improvements the silver in the silver cadmium oxide contacts tend to erode when subjected to endurance cycling. Due to the limited amount of silver present in silver cadmium contacts, erosion of the silver increases the contact resistance eventually causing a situation in which the device fails to trip open after a certain number of cycles.
Some circuit interruption devices which have two pairs of contacts providing a double break can use cadmium free contacts; however, for many applications a single break device is preferred due to certain advantages such a device offers, including higher available contact forces, lower overall voltage drop and lower cost.